The proposed project aims at defining more precisely the mechanism(s) of the unidirectional fluxes of urate in the proximal tubule of rat nephrons. To accomplish this goal, urate transport will be assessed in luminal and contraluminal membrane vesicles from rat renal cortex. Membranes will be isolated and separated by differential centrifugation supplemented by free-flow electrophoresis. Recent studies have demonstrated transport and enzymatic oxidation of urate by the vesicles. The enzyme uricase, a cuproprotein has been found to be responsible for this oxidation. The role of copper, per se, in urate transport has also been evaluated. These studies suggest that urate transport is carrier-mediated, that copper plays a significant role in urate transport and that urate uptake is concentrative in the presence, but not the absence of copper. The present studies are planned to further elucidate the urate transport processes and to examine the role of copper. Since copper is known to alter anion permeance in other epithelia, the relationship of urate transport and anion permeability will be evaluated.